Assessment of health in human faces is context-dependent.

When making decisions between options, humans are expected to choose the option that returns the highest benefit. In practice, however, adding inferior alternatives to the choice set can alter these decisions. Here we investigated whether decisions over the facial features that people find healthy looking can also be affected by the context in which they see those faces. To do this we examined the effect of choice set on the perception of health of images of faces of light-skinned Caucasian females. We manipulated apparent facial health by changing yellowness of the skin: the healthy faces were moderately yellow and the less healthy faces were either much more yellow or much less yellow. In each experiment, two healthy faces were presented along with a third, less healthy face. When the third face was much more yellow, participants chose the more yellow of the two healthy faces more often as the most healthy. However, when the third face was the least yellow, participants chose the less yellow of the two healthy faces more often. A further experiment confirmed that this result is not due to a generalised preference for an intermediate option. These results extend our understanding of context-dependent decision-making in humans, and suggest that comparative evaluation may be a common feature across many different kinds of choices that humans have to make.

Image analysis of weaverbird nests reveals signature weave textures.

In nature, many animals build structures that can be readily measured at the scale of their gross morphology (e.g. length, volume and weight). Capturing individuality as can be done with the structures designed and built by human architects or artists, however, is more challenging. Here, we tested whether computer-aided image texture classification approaches can be used to describe textural variation in the nests of weaverbirds (Ploceus species) in order to attribute nests to the individual weaverbird that built them. We found that a computer-aided texture analysis approach does allow the assignment of a signature to weaverbirds' nests. We suggest that this approach will be a useful tool with which to examine individual variation across a range of animal constructions, not just for nests.

Individual differences in decision making by foraging hummingbirds.

For both humans and animals preference for one option over others can be influenced by the context in which the options occur. In animals, changes in preference could be due to comparative decision-making or to changes in the energy state of the animal when making decisions. We investigated which of these possibilities better explained the response of wild hummingbirds to the addition of a decoy option to a set of two options by presenting Rufous hummingbirds (Selasphorus rufus) with a foraging experiment with two treatments. In each treatment the birds were presented with a binary choice between two options and a trinary choice with three options. In treatment one the binary choice was between a volume option and a concentration option, whereas in treatment two the same volume option was presented alongside an alternative concentration option. In the trinary choice, birds were presented with the same options as in the binary choice plus one of two inferior options. Birds changed their preferences when a poorer option was added to the choice set: birds increased their preference for the same option when in the presence of either decoy. Which option differed across individuals and the changes in preference were not readily explained by either energy maximisation or the decoy effect. The consistency in response within individuals, however, would suggest that the individual itself brings an extra dimension to context-dependent decision-making. This article is part of a Special Issue entitled: Cognition in the wild.

Food preference and copying behaviour in zebra finches, Taeniopygia guttata.

As a social species zebra finches might be expected to copy the food choices of more experienced conspecifics. This prediction has been tested previously by presenting observers with two demonstrator birds that differ in some way (e.g., sex, familiarity), each feeding on a different colour food source. However, if the observer subsequently exhibits a preference, it is unclear whether it has copied the choice of one demonstrator or avoided the choice of the other. Furthermore, this choice may actually be influenced by pre-existing preferences, a potential bias that is rarely tested. Here we examine whether apparent copying or avoidance can be explained by pre-existing preferences. In Experiment 1, observers had the opportunity to watch a conspecific forage from one of the two differently coloured food hoppers. In Experiment 2, the observers did not have this opportunity. In both experiments observers were subsequently tested for their food hopper preference and all but one preferred one colour over the other. In Experiment 1 some observers showed evidence for copying, while others seemed to avoid the colour preferred by the demonstrator. In Experiment 2 females generally preferred the white hopper. Pre-existing colour preferences could, therefore, explain the apparent copying/avoidance we observed. This article is part of a Special Issue entitled: Cognition in the wild.

Physical cognition: birds learn the structural efficacy of nest material.

It is generally assumed that birds' choice of structurally suitable materials for nest building is genetically predetermined. Here, we tested that assumption by investigating whether experience affected male zebra finches' (Taeniopygia guttata) choice of nest material. After a short period of building with relatively flexible string, birds preferred to build with stiffer string while those that had experienced a stiffer string were indifferent to string type. After building a complete nest with either string type, however, all birds increased their preference for stiff string. The stiffer string appeared to be the more effective building material as birds required fewer pieces of stiffer than flexible string to build a roofed nest. For birds that raised chicks successfully, there was no association between the material they used to build their nest and the type they subsequently preferred. Birds' material preference reflected neither the preference of their father nor of their siblings but juvenile experience of either string type increased their preference for stiffer string. Our results represent two important advances: (i) birds choose nest material based on the structural properties of the material; (ii) nest material preference is not entirely genetically predetermined as both the type and amount of experience influences birds' choices.

Context-dependent decisions among options varying in a single dimension.

Contrary to theories of rational choice, adding alternatives to a choice set can change the choices made by both humans and animals. This is usually done by adding an inferior decoy to a choice set of two favoured options that are characterized on two distinct dimensions. We presented wild, free-living rufous hummingbirds (Selasphorus rufus) with choices between two or three options that varied in a single dimension only. The options varied in concentration, in volume or in corolla length. When the options varied in concentration, the addition of a medium option to a choice set of a low and a high concentration caused birds to increase their preference for the high option. However, they decreased their preference for the high concentration option when a low option was added to a choice set of high and medium concentrations. When the options varied only in volume, the addition of a high volume option to a choice set of low and medium options decreased the birds' preference for the medium option. We saw no effects of adding a third option when the options varied in corolla length alone. Hummingbirds, then, make context-dependent decisions even when the options vary in only a single dimension although which effect occurs seems to depend on the dimension being manipulated. None of the current theories alone adequately explain these results.